Electric motors convert electrical power into motion using the force-generating interaction between electrical currents and magnetic fields. Electrical power generators use this same interaction to convert motion into electrical power. A common configuration for both motors and generators is the “brushless direct current” (BLDC) configuration, in which permanent magnets are attached to an axle and surrounded by fixed wire coils. When a wire coil carries a current in one direction, it creates an oriented magnetic field that reverses when current flows in the opposite direction. The coil-generated magnetic fields create a torque on the permanent magnets, thereby spinning the axle. Conversely, spinning the axle causes the magnets to move past the surrounding coils, inducing a current in one direction as the magnetic field increases in one direction, and reversing the current as the magnetic field increases in the opposite direction.
For efficient continuous operation as an electrical motor, a controller switches the currents through the coils in sequence at the same rotational speed as the axle. Such “active” switching has been generally regarded as undesirable in the power generation context because a simple passive (i.e., diode bridge) rectifier generally suffices to extract DC power from a BLDC configuration. However, a passive rectifier limits the maximum power generation efficiency due to non-zero forward conduction voltages. Such efficiency losses become particularly significant for low voltage and/or low speed operation. Thus, it would be desirable to provide a device that enables high-efficiency, low-voltage power generation from a BLDC configuration.